Since its origin, the spread of rice cultivation is extensive and rice is now being grown wherever water supply is adequate and ambient temperature are suitable. The rice grain is covered with a woody husk or hull, which is indigestible and is to be removed in the first step during processing for making the rice edible.
The present book contains cultivation and processing of rice in various ways. The book is very useful for the entrepreneurs, technocrats, research scholars etc.
History, Origin
and Antiquity of Rice
The origin of rice (Oryza sativa L.) has interested
some eminent botanists, and provisional inferences were made in the first half
of this century. A symposium was held in Delhi during 1950 on the origin of
cultivated plants of South Asia, and since then research on the origin and
cytogenetics of rice has been intensified in India and in Japan. Research
publications on the taxonomy, evolution and cytogenetics of rice and its
relatives have appeared in many journals. A recent review by Nayar gives a
comprehensive bibliography and a critical discussion about ‘Origin and
Cytogenetics of Rice’. Some supplementary information is given by Sampath and
these two articles have to be consulted for details. It is here proposed to give
the salient findings and to mention some of the topics on which further studies
are needed.
ANTIQUITY
Formerly,
literary texts as well as traditions were cited to establish the antiquity of
rice cultivation in a particular region. Because of difficulties in
establishing the age of a particular text and in interpreting the statements
pertaining to cereals, archaeological evidence is to be preferred. Where rice
grains, chaff, or husks are detectable in pottery, bricks or mud constructions,
it is possible to identify the material with some confidence and to establish
its age by dating with radiocarbon.
The first
detailed study of an archaeological rice sample from India was from carbonized
grains excavated from Hastinapur, north of Delhi, and was dated as being
between 1100 and 700 B.C. Subsequent archaeological evidence on rice in India
has been reviewed by Buth and Saraswath. They consider the specimens collected
from Atranjikera in Uttar Pradesh to be the oldest found so far and estimate
their period of occurrence to range from 1500-1000 B.C. Vishnu Mittre has given
a detailed discussion about the origin, antiquity and spread of rice
cultivation in India. As regards China, another ancient region of rice
cultivation, pottery excavated from Yang Shao has been found to carry imprints
as well as rice husks. The period of that culture is estimated at 2000 B.C.,
but a greater antiquity has also been claimed. The region covering Burma,
Thailand, and Cambodia is well suited to rice cultivation and has also a large
population of wild rices. Therefore the discovery by a team of American
archaeologists of the most ancient finds of rice from excavations in Thailand
is of interest. The work reviewed by Solheim suggests that these finds reveal
the first agricultural beginning in southern Asia, important enough to be
termed a revolution. The specimens include horticultural plants as well as rice
husks. The dating suggests that these specimens belong to the period ranging
from 5000-4000 B.C. This antiquity may be accepted provisionally and it may be
inferred that rice cultivation spread to Vietnam, Taiwan, China as well as to
India from this centre. However, an independent and parallel origin in Assam,
Bengal and Kerala cannot, at present, be dismissed.
SPECIES ANCESTRAL TO RICE
It has long been
recognized that the wild species of Oryza, closely related to O. sativa, are
widely distributed in India, Burma, Thailand and Cambodia. These wild
populations can be grouped into at least two taxa, but the distinctions are not
clear-cut, as intermediates have been arising as a result of natural crossings.
If the division into taxa is to be made, it is necessary to apply the rules of
nomenclature and decide on the valid names of the two species. This is a
controversial issue; as may be understood from the following account. One taxon
of restricted distribution is found on the margins of ponds, is partly floating
and is potentially perennial. This species is distinguished from its close
relative, which is seasonal, having slender grains and longer anthers, in
addition to some differences in plant and panicle characters. This species was
used to be called Oryza perennis (Moench). This specific name is rejected as
invalid by Tateoka in his comprehensive revision of the genus Oryza. A discussion
about this and allied taxonomic difficulties is included in the book Rice
Genetics and Cytogenetics. In a subsequent chapter of this monograph, details
of taxonomy and species relationship are elaborated. Since the specific name
perennis is widely used and is also convenient to conserve as a valid name,
Sampath published an emended description from a specimen collected in Orissa.
It is, therefore, permissible to consider the Orissa type to be a subspecies of
a widespread, varying and long-anthered wild rice. The other taxon has bolder
grains, shorter anthers and generally stouter awns. This species has large
populations and shows greater variation. Large populations of this species can
be seen beside the railway track on ‘Borrow Pits’ along the east coast of
India, north of Vijayawada and including Bengal. These plants flower during
October when their conspicuous pink awns make the specific name O. rufipogon
Griffith. It is possible that this ‘species’ evolved from hybrids between O.
sativa and O. perennis. Moench emend. Sampath, because it has been repeatedly
observed in many countries that where O. perennis grows adjacent to rice plots,
cross-pollination from wild rice takes place. The extent of crossing is low,
but in the course of time a weed population builds up in the rice fields, since
the hybrid plants shed their seeds, which remain dormant till the next season.
In the course of generations a diversified population can evolve from the
hybrids, and can invade new habitats. It is also possible that the very large
populations may be grouped together as a single species to include genotypes,
which had evolved from O. perennis, before human intervention; as an adaptation
to habitats liable to drought.
A theory has
been advanced that climatic changes during the Pleistocene Period induced
physiological stresses in the herbaceous flora and the evolution of seasonal
forms the existing perennial ones was accelerated. An exposition of this
theory, as pertaining to the Gramineae of Asia, is made by R.O. Whyte (in
press). To apply this concept to rice is to infer the changes as
perennial—climatic stress—seasonal—human selection—cultivated rice. The term
‘genome’, which is explained later in this book has to be used for supporting
this hypothesis. The symbol ‘A’ is used for the genome present in a species at
the diploid level in O. perennis, O. rufipogon and also in O. sativa. The
theory of evolution precludes the separate creation of a species. Therefore the
species having the ‘A’ genome are interrelated and their evolution may be
traceable. A simplified statement of the ancestry of the cultivated rice is as
follows. The perennial long-anthered species is the ultimate ancestor but
possibly another taxon with bolder grains and seasonal habit was the immediate
ancestor. For details, the review of Nayar may be seen. Under this topic, there
is a need for further research to arrive at a firm conclusion.
GENETIC PROCESS INVOLVED IN DOMESTICATION
For the human
selection to operate, there must be genetic variability present in populations,
which must be responsive to the procedures of primitive agriculturists. The
details of cultivation practices in ancient times cannot be traced but it can
be inferred that in some areas, the scrub or the jungle was cut, burnt, crudely
leveled and the seeds of crops were sown. In river valleys and deltas, the
procedures would be slightly different, suggesting a more advanced agriculture.
The method used by primitive agriculturists for harvesting and seed selection
is not known. Initially, the seeds of wild rice must have been used. The
perennial wild rice is partly out crossing, hence, heterozygous and different
populations show differences in genetic composition. Hybridization between
different genotypes, followed by inbreeding, would lead to rapid changes in
plant characteristics. Mutations for nonshedding awnless grains would be
intensively selected by the primitive agriculturists. Sampath has suggested
that hybridity in the molecular structure of some key enzymes could have played
a part in the evolution of O. sativa. Studies on population genetics of the
wild rices of the world have been carried out by Dr H.I. Oka and his
collaborators at the National Institute of Genetics, Misima. These studies
contribute substantially to an understanding of the origin of O. sativa. Two of
his collaborators gave experimental findings and summarized his interpretation.
In a joint contribution the dynamics of plant domestication, as applicable to
rice, is discussed. In view of such significant studies any further advance
under this topic can come only as the result of combined cytogenetic and
biochemical studies on hybrids and hybrid progenies of wild rices.
Breeding
Rice breeding in
India started in 1911 in undivided Bengal, with the appointment of Dr G.P. Hector
as Economic Botanist with his headquarters at Dacca, which is now in
Bangladesh. In 1912, Madras province had the first crop specialist fully
devoted to rice. The period from 1911-1979 may be considered under three
distinct periods as far as rice breeding in India is concerned, viz., of mainly
pure line selections and very few hybridizations of inter-racial hybridization
between japonicas and indices, and of inter-racial hybridization with semi
dwarfs, especially Taiwanese indices.
Prior to 1930,
Bengal and Madras were the only provinces, which had full-time specialists for
the crop. When the Indian Council of Agricultural Research was established in
1929, it initiated rice research projects in many states which did not have a
rice programme and this gave an impetus to the development of rice research in
the country, and by 1950, there were 82 research stations devoted to rice in 14
states of India. These research stations released 445 improved varieties,
mainly by the pure line method of selection. Of course, a few (e.g. ‘Co. 15’,
‘Co. 16’, ‘Co. 25’, ‘Co. 26’, ‘Co. 29’, ‘Co. 30’) were hybrid derivatives from
indica crosses, but numerically they were insignificant when compared to those
evolved through pure line selections. The number of varieties released from
each state is given below.
Ramiah and Rao
have delineated the development of Rice Research Stations in India. The
establishment of these different stations was prompted by the need to cater for
different ecological conditions. Ghose et al. had listed the broad breeding
objectives which made possible the development of 445 improved varieties in the
country. They were: (1) Earliness, (2) Deep water and flood resistance, (3)
Lodging resistance, (4) Drought resistance, (5) Non-shedding of grains, (6) Dormancy
of seed, (7) Control of wild rice, (18) Disease resistance and (9) Higher
response to heavy manuring.
Table 1. The number of varieties
released by different states through selection and hybridization
Thus, the
earlier breeding efforts were directed towards the development of varieties
adapted to specific stress situations or for resistance to diseases prevalent
in the region or what the Japanese called ‘ecological breeding’. When synthetic
fertilizers began to be popular after World War II, efforts were made to
identify varieties which respond to heavy manuring. There were no major pest
problems and the progress though not spectacular did not pose possibilities of
serious disaster. Through pure line selection, the advantages of natural
selections over centuries had been fully made use of and there were no problems
of antagonism involved in the introduction of new genes to an incompatible
environment. The surviving genotypes seemed to be more suited to their
environment underscoring the significance of survival and adaptation in
evolution.
After the
establishment of the Central Rice Research Institute in 1946 at Cuttuck, there
had been a systematic screening of exotic types from the genetic stocks and
many Chinese, Japanese, Taiwanese and Russian types were tested for the purpose
of direct introduction in the country. The result showed that the early
duration local varieties like ‘Benibhog’ were superior to the exotic
introductions. Notable among the Chinese introductions were ‘Ch. 4’, ‘Ch. 45’,
‘Ch. 55’, ‘Ch. 62’, and ‘CNI200 h. 63’, of these, ‘Ch. 45’ proved to be a good
yielder combined with earliness and Helminthosporium resistance and had been
used as a donor in some of the modern varieties.
Prior to 1947,
Chinese varieties were first introduced in Kashmir Valley, possibly due to
reasons of geographical proximity or contiguity and have been found suitable
and so extensively cultivated. The most notable of these introductions is ‘Ch.
1039’ which is the leading variety of Kashmir Valley even today. Others are
‘Ch. 27’, ‘Ch. 47’, ‘Ch. 962’, ‘Ch. 971’ and ‘Ch. 972’.
Though the
Chinese types were fairly successful, the Japanese and Russian introductions
were found unsuitable under Indian conditions, mainly because of their low
yield, unacceptable grain qualities and susceptibility to blast.
Period of inter-racial hybridization between japonicas and indicas
The end of the
Second World War and the subsequent population explosion stimulated the Food
and Agricultural Organization of the United Nations to take up the problem of
improving production of this major Asian and world cereal on an international
basis and the result was a collaborative project of japonica × indica
hybridization in South East Asian countries. Japan had started using chemical
fertilizers from the beginning of this century and so japonicas, the cultivated
rices of Japan, showed response to fertilizer under Japanese conditions up to
60-100 kg N/ha, whereas the indicas, cultivated types in Asia, responded to N
fertilizer only up to 20-30 kg N/ha.
The rationale of
the F.A.O. project was to transfer the high-yielding ability and response to
heavy fertilizer inputs that characterize the japonicas into the local indica
varieties, which were adapted to their respective conditions of culture and had
tolerance to the prevalent diseases and pests of the region.
A parallel
scheme of japonica × indica hybridization was also drawn up by the Indian
Council of Agricultural Research (ICAR) with the same objectives of identifying
varieties with response to fertilizer and having the major features of the
local varieties of the different states.
These two
projects used 192 improved indica varieties, selected by the participating
Asian countries and Indian states and produced a total of 710 different
japonica × indica hybrids. F1 seeds of these hybrids were distributed to the
different participating countries or states for growing the F2 and subsequent
generations in their respective regions to breed varieties suited to those
agro-climatic conditions.
These projects could claim only
very limited success as only four varieties were released from the seven
hundred and odd hybrid combinations. ‘Malinja’ and ‘Mahsuri’ in Malaysia, ‘Adt.
27’ in Tamil Nadu state of India and ‘Circna’ in Australia were the varieties
named.
Another scheme
was lunched by Central Rice Research Institute (CRRI) in 1960 to evolve, high
yielding, fertilizer responsive hybrid varieties with japonica in 11 states.
The development of the semi-dwarf varieties in Taiwan and Philippines and their
introduction into India put an abrupt end to this scheme in 1966, even before
the results could be properly assessed.
But in another
later attempt at Central Rice Research Institute, Rao and Nagaraju achieved
remarkable success in the development of japonica×indica hybrids, fully
achieving the objectives envisaged in the original international and national
hybridization projects. Their success might be attributed to the choice of
short-statured japonicas (as against the tall ones previously used) grown in
South Japan, which climatically is fairly similar to ‘Taiwan’ (and not from
Hokkaido, the coldest region where rice is grown). So, varieties adapted to
mild temperate region were seen to be more productive under tropical conditions
than those from extremely cold temperate zone. This emphasises the importance
of selecting suitable parents with adaptability in rice
improvement/hybridization programmes.
During the
period of japonica×indica hybridization, time and again it was stressed, that
the japonicas had high-yielding ability and response to fertilizer. But in
India, the introduced japonicas had been a total failure, except in the hills
and some cool areas. Japonicas were both photoperiod and temperature sensitive
and so flowered in 35-40 days and did not get enough time for proper vegetative
growth and tillering and so were not half as productive as the indicas under
Indian conditions nor did they exhibit any of the virtues for which they were
famous in Japan. Therefore, the limited success of the first two japonica × indica
hybridization projects was natural as the very premise of the project of
transferring the high yield potential and response to fertilizer of japonicas
was not apparent in them under Indian conditions. Besides, the ‘character’ of
response to high fertility is an interaction of environment and genotype and
when the environment was changed the interaction also gave different or
negative results. The chances of getting hybrid recombinants with the desirable
attributes of both the parents from such a wide genetic scrambling were a
slender as getting highly productive hybrids as transgressive segregants from
any other inter-or intra-racial crosses involving ordinary or poor yielding
parents.
It was obvious
that the short photoperiod and tropical conditions of the Indian plains,
transformed the entire physiology of growth, development and productivity of
japonicas, which therefore could not provide productive recombinants in a
Mendelian proportion. The ecological specialization to divergent situations had
caused genetic incompatibility between the races and the japonica × indica
hybrids were seen to have a very high degree (even to 99%) of spikelet
sterility in the segregating populations. This is an interesting instance of
interaction between genotype and environment ruining the genetic potential for
productivity in crop plants themselves or in their hybrid derivatives.
In Japan, during
the rice season, the days are longer and there is a higher level of solar
radiation that in tropical countries. In tropical region of India, the day
length is fairly constant during the crop season, but with low solar radiation
due to the overcast sky of the monsoon period. Where the long duration crops
are raised, though the days are bright, there is a shortening in day length, after
the autumn equinox, contributing to a reduction in the availability of per day
solar radiation. This is one of the significant differences between the rice
growing environments of tropical and temperate regions.
As indicated
earlier, CRRI has been exploring the possibility of direct introductions of
exotic types from leading rice producing countries like Japan, Taiwan, etc.
Some of the Japanese varieties, when tried under 90 kg N and 35kg P2O5 per ha
were found promising (though on par or inferior to local varieties in yield)
especially ‘Norin 17’, ‘Norin 18’ and ‘Zuiho’. The Taiwanese introduction
Hsunchu was found not as productive as the local or Japanese types. The
subsequent introduction of ‘intermediate’ types from Taiwan proved successful
in many parts of the country like ‘Taichung-65’ in Karnataka, ‘Taichung
(Native) 1’ in Bihar, ‘Tainan-3’ and ‘Kaohsiung-18’ in Kerala and ‘Hsunchu’ in
U.P., almost setting the stage for the next phase in Indian rice breeding.
Period of inter-racial hybridization between semi-dwarf Taiwanese
types/derivatives and indicas
The development
of ‘Taichung (Native) 1’ from the semidwarf mutant Dee-geo-woo-gen was major
event in rice research in Asia and particularly for India, ‘T(N) 1’ recorded a
productivity which was considered impossible in the tropics before. It was felt
then, that through extensive cultivation of non-lodging semidwarf hybrids, rice
production could be substantially increased in a short time as in wheat.
Enunciation of the plant type concept, from an elaboration of the morphology in
terms of the physiological efficiency of the semidwarfs, stimulated breeding
activity throughout most of South Asia and especially India which operated its
most intensive rice breeding programmes, since 1965, under the All-India
Co-ordinated Rice Improvement Project (AICRIP). Initially, the aim was to
identify semidwarf varieties that would yield well from Kanyakumari to Kashmir,
so as to make the seed multiplication and distribution system effective.
‘Padma’ and ‘Jaya’ were the first varieties that emerged from this programme.
Subsequently varieties were released by Central Variety Release Committee, and
by the different state agencies. The list of released varieties is given in
Appendix I. The numerical superiority of state releases stresses the importance
of regional adaptation in rice varieties. Most of these varieties have a yield
potential of 3-5 tonnes/ha.
The most
significant aspect of this period is the prolific release of hybrid varieties.
During this phase, 123 varieties were released in twelve years, compared to theNI200
51 hybrid varieties released during the four decades prior to 1965. This surge
in hybrid releases was facilitated when semidwarf plant habit became one of the
easily identifiable selection criteria for breeders.
The plant type
or semidwarf varieties with the genetic architecture for physiological
efficiency of grain production have been found to be superior to the tall
traditional varieties in both kharif and rabi seasons, but more so in the rabi
season. The following table illustrates the superior response of semidwarf
varieties to nitrogen inputs for grain production in comparison to the
traditional varieties during rabi season when the cultivation is under
controlled irrigation and ample solar radiation.
As with japonica
× indica hybridization, the inter-racial hybridization programme with Taiwanese
varieties or derivatives also ran into difficulties. It was unfortunately
reported in the early phase of the semidwarf period that through adoption of
semidwarf varieties with improved management practices, the production problem
could be solved, as was done in wheat. But rice being cultivated during the
monsoon, when no other cereal could be grown in heavy rainfall areas normally,
faced problems of adaptation to specific ecology and the newly-introduced
semidwarf types were found unsuitable in a variety of stress situations, such
as water logging, salinity, drought, low solar radiation due to clouded
atmospheric condition, etc. when these semidwarf varieties were cultivated
under high fertility conditions, they were found susceptible to most of the
pests and diseases of rice. Continuous and intensive cultivation of these
semidwarfs caused disease and pest epidemics, which gave premonitions of famine
or ruin as in Bihar, Andhra Pradesh, Kerala, Indonesia and elsewhere. These
facts again stress the importance of adaptability in monsoon rice varieties to
the tracts in which they are to be grown. It is well known that monsoon fosters
most of the pests and diseaseNI200 s of rice and high levels of fertilizer inputs
aggravate their intensity. In such a situation, it is unwise to advocate
varieties of identical genetical constitution over wide tracts. Genetic
diversity is still the best insurance against disease and pest epidemics as is
illustrated by the Indonesian and Kerala catastrophes.
Table 2. Grain yields of
semidwarf and local types in kharif and rabi under different nitrogen levels
The concern with
disease and pest epidemics has intensified efforts for incorporation of
‘multiple resistance’, by which is meant resistance to more than one disease or
pest, in the varieties to be developed. Many of the traditional indicas have
been found to be the major donors for disease and pest resistance.
Thus, having
implicitly accepted the production superiority of the semidwarfs and widely
popularized them, we have to embark on breeding ‘plant-type’ varieties with
tolerance to physiological stresses like drought, water-logging, saline
tolerance, cold tolerance, resistance to diseases and pests and good cooking
and eating qualities, rather to transfer the desirable traits of the local
varieties to the ‘plant-type’ background. The major efforts made in these
directions are summarized below.
Breeding upland rices with tolerance to drought
In monsoon
dependent rice cultivation, uplands with rainfall of 700-1100 mm get exposed to
moisture stress periodically, due to breaks in monsoon lasting for different
periods of a week to ten days of erratic distribution of rainfall. Such areas
constitute about a sixth of the world’s rice acreage and third of the kharif
rice area in India and it is necessary to stabilize yields from such lands to
keep up the upward trend in rice production.
Uplands are
defined as those lands which are not bunded and wherein water is not therefore
impounded during cultivation. Upland rice cultivation entirely depends on
rainfall and it is a way of harvesting rain by adopting varieties of suitable
duration according to the rainfall pattern.
Four kinds of situations are possible
for such a kind of rice cultivation:
• Rains adequate or assured during
vegetative and reproductive phases;
• Rains inadequate or unreliable during
vegetative phase but adequate during reproductive phase;
• Rains adequate during vegetative phase
but inadequate during reproductive phase;
• Rains
inadequate during both vegetative and reproductive phase;
The crux of the
problem in upland breeding (exposed to moisture stress) as in items, and is to
find out suitable donors with drought tolerance during the vegetative and
reproductive phases as under situation in item, rice cultivation is not
possible and in item there is no problem of moisture stress.
As there are
uplands in all the rice growing states, many local varieties suited to such
conditions have been identified. Through screening, a number of varieties with
varying degree of drought tolerance have been identified (e.g. ‘Mtu. 17’,
‘Mettasannavari’ from Andhra Pradesh, ‘Ch. 45’ from Bihar, ‘Sathi-34-36’ from
Gujarat, ‘Ptb. 28’, ‘Ptb. 29’, ‘Ptb. 30’ from Kerala, 'B-76' from Orissa,
'Lalnakanda-41' from Punjab, 'Tkm.l' from Tamil Nadu and ‘N. 22’ and ‘Sudha”
from Uttar Pradesh.
Among these, it was found that
‘Lalnakanda-41’ 'Ch. 45' and ‘N. 22’ have drought tolerance at the vegetative
phase, while ‘Mtu. 17’ showed drought tolerance even at the reproductive phase.
The first
attempt recorded to breed varieties with drought tolerance was in Tamil Nadu
during the mid-fifties and a drought-tolerant variety ‘Co. 31’ was released.
Kerala also reported some drought-tolerant breeding lines from the cross
Krasnodar × Kattamodan, Culture No. 356 especially.
With the
introduction of ‘Taichung (Native) 1’, during 1965, efforts were made to
transfer drought tolerance to semidwarf hybrids, and ‘Bala’ from the cross ‘N.
22’ × ‘T (N) 1’ was the first high-yielding variety with drought tolerance that
was released in the country. As ‘Bala’ was hard threshing, efforts were made to
identify lines with easy threshing and good grain qualities. ‘CR. 113’, ‘CR.
115’, ‘CR. 141’, and ‘CR. 143’ had many lines with better threshability and
grain qualities than ‘Bala’. One line, viz. ‘CR. 141-192’, from the cross (N.
22/ T(N) 1 × T. 90, IR. 8) had been named ‘Kiran’ in Bihar. Hybrids more
productive and tolerant to drought than any of the parents had been identified
in the cross CR. 125 (Lalnakanda-41 ×Mtu. 17) × T(N) 1.
International
Rice Research Institute (IRRI) geneticists have standardized the testing
procedure for drought tolerance of upland rices and have made considerable progress
by evolving a large number of promising cultures suited for uplands. Many of
these are tested in most rice growing countries including India through the
International Rice Testing Programme (IRTP). As the upland rice problems are
faced by every rice-growing state in India, a good number of cultures have been
generated by states using local donors, and at present, many are under trial in
the AICRIP testing programme.
Efforts were
also made at Central Rice Research Institute (CRRI) to evolve varieties with
drought tolerance through induced mutation in traditional varieties.
Considerable success was achieved through this approach and many mutants with
higher yield potential and drought tolerance than the parents had been
identified in ‘Ch. 45’ and ‘Mtu. 17’. Mutant Number 2 and 12 of ‘Mtu. 17’ had
been in district trials in Meghalaya and Manipur. Of special significance is
‘Mtu. 17’ Mutant No. 4, which showed very high tolerance to drought even during
the flowering phase.
Another mutant
from ‘CR. 113’, designated ‘CRM. 13-3241’ is possibly the earliest induced
productive major cereal in the world, maturing in seventy days when direct
seeded under a temperature regime of over 25°C. This mutant yields about 1½-2
tonnes/ha normally but with good management has shown potential up to 5
tonnes/ha. In many State Farms under the Department of Agriculture of Orissa
Government, it had recorded yields 2½-3½ tonnes/ha. By relying on the earliness
of this variety, known or predictable drought spells can be avoided or there is
a possibility to raise another rice crop after the drought or flood ravages and
the resumption of normal monsoon. This variety is to be named shortly by the
Orissa Department of Agriculture. In Assam, it is found to be promising as
pre-flood kharif variety (March-June) suited for direct seeded condition where
it could be grown with the rains received during March-June. In Tripura, it has
been found to be useful in ‘Tillo’ lands (low mounds). This mutant is under
extensive trial in West Bengal. Arunachal Pradesh and Madhya Pradesh.
Breeding for water-logged and lowland conditions
Kharif is the
main rice crop or season for India, extending from June to December,
practically coinciding with the onset of the south-west monsoon and complete
recession of north-east monsoon. Of the total 38.9 million hectares under rice
in India, about 20 million ha or 50% of the area are under lowland where there
is standing water of varying depths depending on the topography of the land for
varying periods. The lowlying areas can be classified into:
Water-logged area (ill-drained
conditions),
Flooded areas, and
Deep water areas.
The water-logged
lowlands can be grouped into four categories depending on the depth of standing
water, and the approximate area under each according to the type of cultivation
is shown in the following table.
The above
classification is mainly based on the toposequence of rice fields. With the
onset of monsoon, medium lands have shallow rainfed conditions but water gets
accumulated later at the peak of monsoon. So in the high rainfall zones, medium
duration photosensitive varieties are grown in such lands. Where the rainfall
is low, photosensitive varieties, which flower in 100-110 days, are preferred.
The intermediate lowlands constitute about half of the water-logged areas and
photosensitive varieties are grown in such lands. In the semi-deep and deep
water areas, there is stagnation of water with the onset of heavy rains
(normally from mid-July onwards) and there is no way to drain off inundated
water. Under such situations only broadcasting with the onset of monsoon is the
usual practice.
Table 3. Distribution of
water-logged areas according to type of cultivation and photosensitivity in
million hectares
Soils-Their
Classification and Agro-Chemical Characteristics
The soils on
which rice is grown are so extraordinarily varied that there is hardly any type
of soil on which it cannot be grown with some degree of success. It is,
however, necessary that the deficiencies of the various soils are identified
and made up to increase their productivity.
Classification and Distribution
The soils on which rice is grown in India and their classification
The major soil
groups producing rice are: Riverine alluvium, red-yellow, red-loamy, hill and
submontane, tarai, laterite, coastal alluvium, red sandy or gravelly, patches
of mixed red and black, medium and shallow black soils.
The soils can generally be
classified for purposes of rice cultivation in India into:
1.
Alluvial soils (Haplaquents, Ustifluvents, Udifluvents, Haplustalfs,
Ustochrepts),
2. Calcareous alluvial soils
(Calciorthids),
3. Coastal and deltaic alluvium
(Propsualfs),
4. Red soils (Paleustalfs, Rhodustalfs,
Haplustalfs),
5. Red and yellow soils (Haplustults,
Ochraqults, Rhodustalfs),
6. Lateritic soils (Plinthaqults,
Plinthustults, Plinthudults, Oxisols),
7. Black soils (Ustochrepts, Uatropepts,
Pellusterts, Chromusterts, Pelluderts).
8. Mixed red and black soils (association
of Alfisols and Vertisols),
9. Grey-brown soils (Calciorthids),
10. Brown hill soils (Palchumults),
11. Submontane soils (Hapludalfs),
12. Terai soils (Haplaquolls),
13. Desert soils (Lithic Entisols,
Psamments, Calciorthids),
14. Saline-alkali soils (Salorthids,
Salargids and Natrargids), and
15. Peaty and saline peaty soils
(Histosols).
Table 2. Ranges of moisture index
and the mean annual temperature in the various climatic zones, as used by the
Co-ordinated Agronomic Experiments Scheme
Table 3. Characteristics of the
agroclimatic regions of India
For a
comprehensive and meaningful development of research programmes on a regional
basis, the Indian Council of Agricultural Research has identified eight
agro-climatic regions in the country, and these regions also represent the
typical rice-growing regions of the country. The agro-climatic regions
encompassing the different states with soils, rainfall, temperature, etc.,
which are significant from the point of view of rice cultivation are given in
Table 1. This broad division into general agro-climatic regions is suited for
general agricultural purposes. The soils are also subdivided into agro-climatic
regions based on the degree of wetness, as measured by moisture index, which is
the excess of precipitation over the potential evapotranspiration, expressed as
a percentage of the potential evapotranspiration divided into 8 classes,
designated one to eight, with increasing wetness and with each one of them
again divided into subclasses, A, B, C, D, and E, which are in an ascending
order of coolness, based on the mean average temperature. The ranges for the
various classes are shown in Table 2. This classification, as used by the
Co-ordinated Agronomic Experiments Scheme, might be very useful for determining
the cumulative effect of climate on soil characteristics, but for its direct
effect on rice growth, the regions were divided into ten climatic zones by
Ghose, Ghatge and Subrahmanyan, not only depending on the rainfall, but also on
the critical temperature in the cold months, the duration of the dry periods,
relative humidity, etc., as described for the individual states in the last
section of this chapter. The characteristics of these zones are shown in Table
3.
Distribution of various kinds of soils in India
The state-wise
area under rice is given in Table 4. The area occupied by rice in West Bengal
and Bihar is nearly the same, followed by Orissa, Madhya Pradesh, Uttar
Pradesh, Andhra Pradesh, Tamil Nadu, Assam, Maharashtra and Karnataka. These
states put together, account for more than 90 per cent of the total
rice-producing area. They also constitute the traditionally rice-growing areas
in the country. The rest of the states have, however, limited areas under the
rice crop.
The humid western Himalayan region. This region
comprises submontane soils, hill soils and terai soils in the states of Jammu
and Kashmir, Himachal Pradesh and the Kumaon and Garhwal divisions of Uttar
Pradesh.
The soils that
are found in the rice-growing tract of Jammu and Kashmir are formed from the
alluvium brought by the major rivers Chenab, Ravi, Tawi and their tributaries.
They occur mostly in the Jammu and Kathua districts. They vary in depth, are
light in texture and their pH ranges from 6.5 to 8.7, they are high in organic
matter, nitrogen and K2O, but are deficient in phosphorus.
The submontane
soils include the valley floor and the karewa soils which occur in the
Anantnagh, Baramulla and Srinagar districts. The valley floor has been
constituted by the alluvium deposited by the Jhelum and the Indus. They are
silty loam to clay loam and are neutral to alkaline (pH 5.4-8.5).
The karewa soils
are somewhat eroded and formed from the deposits, which are of lacustrine
nature. Their texture is heavy; their contents of nitrogen and organic matter
are moderate to high, and their total P and K ranges from 0.09-0.3 and 0.1 to
0.2 per cent respectively.
The hill soils
occur in Uttar Pradesh in the districts of Almora, Chamoli, Pithorgarh, Uttar
Kashi and Dehra Dun. They are shallow with fragments of rock occurring within a
few centimeters at higher elevations but about three meters in valleys and
lower depressions. They are derived from biotite schists and phyllitic
materials under moist conditions. The soils groups described by Mukherji and
Das fall under the categories of red loam, brown forest soil, meadow soil and
podzolic soil.
The terai soil
occurs as a narrow strip from the north-west to the extreme north-east. The
soil remains saturated throughout the year because of sufficient precipitation
and high ground water-table. They have been formed from the transported
materials laid down by different rivers originating from the Himalayas. They
are productive and respond to fertilizers. They are classified as Molisols in
soil taxonomy.
In Himachal
Pradesh, the hill soils are formed over a variety of parent rocks comprising
sandstones, gray micaceous sandstones and shales in the sub-Himalayan region
where they are located. The soils are loam to silty loam and medium to high in
organic matter, total nitrogen, phosphorus and potash. They are poor in
available nutrients. The cation-exchange capacity is low to medium.
The humid Bengal
Assam basin and the humid eastern Himalayan region and the Bay islands. For
convenience, these two regions are dealt with together. The altitude of the rice-growing
areas ranges from a few metres in Sundarbans in West Bengal to about 1660
metres in the north-eastern part of the Himalayas in the Mizoram State and up
to more than 2,000 metres in Arunachal Pradesh.
The crop is
often grown on flat lands to facilitate the supply of water needs. It is grown
successfully over a wide range of slopes, ranging from nearly level to very
steep (podu or thum) cultivation in hilly areas. One of the main limiting
factors is the availability of water.
Owing to the
adaptability of the rice crop to soils having a wide range of characteristics,
it is not possible to categorize a particular soil group as rice soil or assess
its best use as rice land.
The major groups
of soils listed in the table for the two regions included riverine alluvium,
the terai soils, red loamy, sandy, or gravelly, red-yellow and laterite soils.
Some of the important soil series cultivated for rice in West Bengal extracted
from the Soil Survey Reports are Canning, Kharbona, Jagdishpur, Sasanga, Hanrgra,
Totpara and Banpara. They are placed in Entisol, Inceptisol and Alfisol in soil
taxonomy.
The alluvial
soils deposited by the rivers mostly occupy the major part of the wetland rice
soils, thus contributing the largest share to rice production in the country.
They are derived from the deposition, mainly as silt deposited by the numerous
tributaries of the Ganges and the Brahmaputra systems. The different weathering
products of the Himalayas are deposited during the course of their flow through
the plains.
In the wetlands
the water-table is high, the drainage is poor and the entire profile remains in
a reduced state. Mottled horizons are common and the accumulation of calcium
carbonate in the lower horizons is also observed in soils. The flooded
condition of paddy soils brings about the movement of iron and manganese
compounds from the upper layers and their precipitation in the reduced zone of
the lower horizon.
In West Bengal
the ‘Rarh’ region which comprises portion of Murshidabad, Bankura, the whole of
Burdwan and the western half of Midnapore are classified under old alluvium.
According to Mukerjee et al., and Digar, the textures vary from sandy loams to
heavy clays with a hard pan.
The laterite and
lateritic soils are found between the Damodar River and the Bhagirathi River,
interspersed with basaltic and granitic hills. They may be classified into two
groups. The first group consists of soils of Midnapore, Bankura, Burdwan and
Birbhum. In these soils, the ratio of SiO2: Al2O3
is quite high and because of chemical weathering, followed by considerable
leaching, the soils are deficient in N, P2O5 and K2O.
They respond to N and P fertilization. At some places, buried laterites are
also observed at considerable depths underlain by alluvium. These soils give better
response to P2O5 and the yield of rice is significantly
increased by the application of P2O5 rather than by that
of N.
The red soils of Birbhum, Bankura, Burdwan and West
Dinajpur sometimes misclassified as laterites are transported from the hills of
Chhotanagpur Plateau. They are acidic, poor in Ca, N and available P. They are
highly leached and respond to N and P.
The coastal
soils in the districts of 24-Parganas and Midnapore after reclamation are
producing good crop of rice. They are also rich in plant nutrients. The terai
soils in the Jalpaiguri and Cooch-Behar districts lying at the foot of the
Himalayas are of raw humus type, sandy and gray to black.
Soils in the
Assam Valley are acidic, specially the old alluvial soils, whereas the new
alluvium is slightly acidic to neutral and, in some cases, slightly alkaline.
The soils are high in available P and K and moderate in organic matter and
nitrogen.
The lateritic
soils occur in the north-eastern mountainous upland areas of Assam. Drainage in
the uplands is good. The groundwater laterites are poorly drained. In some
parts of West Bengal, by the augmentation of irrigation sources from
groundwater reserves through the sinking of tube-wells, rice is grown in low,
medium and upland situations. Though rice is adapted to a wide range of soils,
as mentioned earlier, the type of soils suitable for it mainly depends upon the
conditions under which the crop is grown rather than upon the nature of the
soil.
By the increase
in demands for more areas to be brought under the rice crop, the conservation
of moisture during certain periods becomes necessary owing to insufficient
irrigation water. Therefore effective soil depth and suitable texture are very
important.
In wetland
cultivation, soil structure is of little significance, but good soil structure
ensures better water transmission and moisture preservation for the dryland
crop.
The rice crop is
better grown mostly in acidic soils whose pH ranges from 5.5 to 6.5. It is
successfully grown in saline soils of Sudarabans in the Gangetic delta.
The sub-humid Sutlej-Ganga alluvial plains
This region
experiences low winter temperature and the usual practice are to take a single
crop of rice between May-June and September-October.
The major groups
of soil growing rice in the region are calcareous alluvial, riverine alluvial,
saline-alkaline, red-yellow loam, red sandy or gravelly and mixed red and
black. The alluvial soils owe their origin from the materials brought and
deposited by the great rivers from the mountains. They are rich in potash and
calcium, but are deficient in organic matter, nitrogen and phosphorus. The
older alluvium is generally deficient in phosphorus, lime and organic matter,
whereas the recent alluvium is well supplied with nutrients because of fresh accumulation
of river silt. The soils are placed in Entisol, Inceptizon and Alfisol
categories.
In the irrigated
tracts of the Punjab state, the soils are light-textured and alkaline. Organic
matter and nitrogen are low.
In Uttar
Pradesh, the alluvial soils occupy nearly 60 per cent of the area in the east,
west, south and central parts of the state. They have developed from the
alluvium deposited by the Ganga and the Yamuna and their tributaries. The soils
can be broadly classified under (1) light-textured alluvium of the west and
north-west, (2) alluvium in the centre possessing intermediate textures, and
(3) alluvium in the north-east derived from the calcareous parent material.
Saline and
karail soils occur all along the Ganga river on the left side in the districts
of Meerut, Aligarh, Bulandshar, Manipuri, Etah, Kanpur, Fatehpur, Allahabad,
Lucknow, Pratapgarh and Sultanpur. The parent materials are alluvial deposits
in the riverine areas and finely washed materials in the lower depressions. The
soils are highly alkaline, indurated and have hard pan which obstructs the
downward movement of water.
The karail soils are black, finer in texture and
occur in the lower basin of Ganga. They occur in the districts of Allahabad,
Varanasi, Ghazipur and Balia. They are formed from the black alluvial deposits
transported by the Yamuna from central India.
The Ganga
divides Bihar into two halves, north and south. The alluvium north of the Ganga
has texture varying from sandy loam to clay loam and the pH is neutral to alkaline.
Alkaline soils are generally found where the lime content is high. The alluvium
south of the Ganga, comprising the districts of Patna, Ganga and parts of
Shahabad, is gray to black, and light loam to heavy clay. Lime is less and soil
pH is slightly alkaline, changing to the acidic range in the southern
extremity. The middle part which lies in a depression gets flooded during the
monsoon. The available K2O and P2O5 are high.
The red soils
occur in the districts of Ranchi, Hazaribagh, Santal Parganas, Singhbhum, and
Manbhum. They are acidic (pH 5.0-6.8) and contain higher and soluble Fe2O3
than Al2O3. They are rich in available K2O but are low in
P2O5.
Seed Rice and Seed Production
The wide use of
newly released varieties and proper seed production from breeder and foundation
seed to the growers’ seed stock on the farm are essential for high-level rice
production with minimum input. New and superior varieties, however, can make
their contribution to practical agriculture only if the seed reaches the farmer
in varietally pure state, in adequate quantities, in an undamaged condition,
free of weed seed, and at a reasonable price.
The general
purpose of seed production is to increase those old and new varieties, which
are superior to standard varieties for commercial distribution. The production
of seed rice consists of growing the primary seed, called foundation seed, and
then increasing this seed in sufficient quantities to meet the request of the
practical farmer for his seed stock supplies. To produce high-quality seed, a
grower must have a superior seed source of a well-adapted variety. Formerly,
each farm would obtain a certain amount of such seed and multiply it to
establish its own seed stock on the farm. But today, modern harvesting and processing
methods, bulk drying and storage have increased the possibility of seed mixing.
This led to the need for sources of pure seed. As a result, the seed
certification program now in effect in this county is an important part of rice
production.
Sources of Pure Seed
Production of
primary seed is carried out by institution for rice research and their
experimental stations and farms. They produce foundation seed (super-elite and
elite) and multiply promising varieties for release to the growers. Production
of farm seed stock is done largely by the commercial grower who breeds
foundation seed through three generations, the third of which is sown for
commercial grain output.
Classes of Seed
The classes of seed, termed
breeder, foundation and certified seed, can be described as follows.
(1) Breeder seed is seed directly
controlled by the plant breeding institution, and is the source of select seed
handled at selected nurseries for the production of seed of the certified
classes.
(2) Foundation seed is the progeny of
breeder or select seed handled at seed increase nurseries to maintain specific
genetic purity and identity. Foundation seed is usually the first-year increase
from breeder seed. It is produced on fields that have not grown another variety
or a lower class of the same variety during the 2 previous years. The
distribution of foundation seed to growers usually is handled through
specialized seed production farms and/or stations under a breeding center that
increase this seed to the commercial growers as a certified class seed. For new
varieties or for old varieties in short supply, specified amounts of seed may
be increased or reduced depending on demand.
(3) Certified seed is the progeny of
breeder, and more so that foundation seed is handled so as to maintain a
satisfactory level of genetic purity and identity. It is produced in riceland
areas specifically allotted for seed increase purposes. The production and
certification of seed is not a part of the breeding program.
The super-elite
and elite seed is distributed to growers to be increased to quantities
sufficient to maintain a seed stock necessary to satisfy the grower’s needs.
Usually, the third-year increase from certified or foundation seed is used for
commercial grain production. Thus, seed of a commercially established variety
is renewed once in three years. For the production of the various classes of
certified seed it is necessary to have clean land and to prevent mixtures in
seeding, harvesting, and processing. The careful tending of all fields to
remove undesirable weeds, other crop and off-type plants may increase the
production costs, but is very essential.
Seed Rice Culture
Varietally pure,
high-quality seed in a viable condition can be obtained only through the proper
use of the whole spectrum of agronomic practices. This includes adequate
seedbed preparation, crops grown preparatory to seeding rice, the use of
high-quality seed, optimum dates and methods of seeding, adequate
fertilization, and, finally proper mechanical treatments (threshing, cleaning
and grading). Practical experience has indicated that seed rice grown in good
soil that receives the best fertilization and cultivation treatments is usually
larger in size than seed of the same variety grown in poor soil and
inadequately cultivated. The higher the level of cultivation, the slower the
process of varietal deterioration under commercial farming. Strict observance
of the seed production cultivation requirements usually results in seed with
high varietal and field qualities which will be preserved well in the 5th, or
even 6th, generation. Any retreat from the established seed-rice cultural
requirements may bring about a rapid deterioration in the quality of even the
first-year seed. This will undoubtedly reduce the grain output of table rice in
the area.
Usually, the
fields where rice will be grown for seed are treated much better than the
commercial rice paddies to benefit the rice grower with seed rice of high
standard. To avoid mixing, each variety is sown with a separate clean seeder.
The results of rice research and advanced practice indicate that perennial
grasses, cultivated fallows, and new lands developed for rice are good for seed
rice production. The land should be thoroughly worked to a fine tilth and
adequately fertilized. Saline lands are considered inadequate for seed
production and should be avoided. The irrigation and drainage facilities should
be operable and in good shape, and the land levelled to allow rapid flooding
and draining if necessary.
The best time to
sow rice for seed is when the soil temperature at a depth of 3-5 cm is 14 to
16°C, which for most rice-growing areas occurs in late April and early May.
To obtain a high germination rate
the seed usually kept cooled during storage in the winter period, is aerated
and warmed up either in grain bins or grain driers, and treated with granosan M
2-3 weeks before seeding.
The rate of
seeding depends on the variety and may vary from 4.5 million to 6 million
viable seeds per hectare. Where the elite seed is being increased for
commercial release, the rate of seeding is reduced to 4-5 million viable seeds
per hectare. Good results can be obtained by drilling 3 million viable seeds
per hectare in rows spaced at 30 cm. This method has proved effective for rapid
multiplication of new and promising varieties since under such a wide-row
method of seeding the multiplication coefficient increases enabling the grower
to achieve higher yields at a much lower rate of seeding. Under this method, a
rate of 100 kg viable seeds per hectare gave 7.16 t/ha of seed rice according
to the USSR RRI data.
The wide-row
method of sowing rice for seed provides for a uniform ripening of seeds on the
main and lateral panicles, improves plant resistance to lodging and increases
the productivity of the plant stand. All this in turn reduces the risk of blast
disease and produces seed of a higher class. In addition, this method allows
for easy weeding to maintain varietal purity and identity of seed by removing
off-type and other crop plants from the field.
The time to sow
rice for seed is equally important. Early seeding results in a thinned stand
establishment during emergence, while with delayed seeding the seed usually
fails to fully mature and, as a result, exhibits poorer germination.
Seed rice
plantings require optimum levels of nutrients, particularly phosphorus.
Excessive applications of nitrogen fertilizers should be avoided because high
nitrogen contents delays maturity, especially when the weather during the growing
period is cool and rainy. In addition high nitrogen weakens the strength of the
stem of the rice plant, which leads to severe lodging, which results in poorly
filled grain, high spikelet sterility, problems at harvest time, and
germination in the panicle.
Insofar as
possible, seed fields should be managed so as to minimize lodging and produce
satisfactory yields without excessive vegetation growth. This is impossible
with high single rates of nitrogen, which must be applied in divided or split
dressings. In seed fields ammonium sulfate and urea are preferred over all
other sources of nitrogen.
Phosphorus
fertilizers appear to improve seed quality. Depending on the forecrop and
degree of soil salinity, phosphorus is applied as basal at rates from 90 to 150
kg P2O5 per hectare before seeding. Potash is also essential for seed fields to
facilitate maturity, obtain well-filled grain, and reduce the percentage of
empty spikelets. Potassium is usually applied as topdressing during leaf-tube
formation (the 8-9-leaf stage) at 30 to 60 kg K2O per hectare.
The Control of Red Rice
The uses of specific varieties
that differ in maturity, grain type, processing and cooking qualities of rice
grain have increased the possibility of seed mixing. In this respect, the production
of seed that is varietally pure and free of persistent weed seeds become
extremely important. Preventing intermixing throughout the various phases of
seed production requires very close attention by the grower. Commercial
varieties could become badly mixed with other varieties and infested with weedy
strains of rice. These strains are the red rices that reduced grain and milled
yield during harvesting and processing.
All the strains
of red rice are characterized by severe shattering, rapid growth, high yield,
and a tolerance to adverse environments. Red rice produces many tillers (up to
60), and the progeny from one seed may amount to 1500-1600 viable seeds.
Usually, the grower inadvertently spreads red rice by planting contaminated
seed. Because herbicides do not selectively control red rice in the rice crop,
infestations should be removed from seed rice fields by other methods if one is
to avoid deteriorated quality in seed rice and prevent further spreading of the
weed. Red rice contaminates not only the seeding material but also the soil.
Tests have indicated that without proper weeding, the quantity of red rice in
the seeding material the following season increases 5 to 10-fold.
To control red
rice it is necessary to know the biology of its strains. Control is difficult
yet possible through crop rotations, weeding operations, renewal of seed
sources, adequate tillage, etc. Red rice infestations of soil can be prevented
through using land cropped with perennial grasses, seeded fallows and new riceland
for elite propagation and seed rice fields. Red rice plants that appear in the
first year alfalfa crops following rice do not produce seed because they are
cut out with each cut of alfalfa for hay.
Red rice seeds
shed into the soil remain viable for several years, and are able to sprout from
a soil depth of 10 cm. Thus the emergence of a red rice seed plowed under in
the fall to depths of 2 and 10 cm would be 20 to 10 percent, respectively. All
plants that emerged would develop well and produce seed.
Flooding or
flushing the soil to provoke red rice emergence is an effective means of red
rice control. The method is particularly useful in cultivated fallows where a
flood is established after the fallow-grown crop has been harvested to soak the
soil to refusal. The weeds and volunteer rice plants are then killed by disking
or working the field over once with a chisel or plow. Besides mechanical
eradication of the soil-borne red rice, use of high-quality seed rice that is
free of red rice and other weed seeds is an effective way of controlling
repeated infestations.
Red rice
infestation increases without regular rogueing of seed fields, or when rice
follows rice continuously. Infestation will also increase if the grower relies
on his own seed stock for several seasons, or if the seeding material is badly
mixed.
Seed rice fields
should be rogued several times during the last part of the growing season to
eliminate not only the red rice plants but also the mixed varieties or rogues.
The first rogueing is done at tasseling when the panicles of the early rices
are visible. The second rogueing is initiated when the seed rice variety has
fully developed and the rogues can be checked for the absence or presence of
awns and colouration of the panicles. All awned plants are then removed from
the seed fields growing awnless varieties of rice and, conversely, all the
awnless plants are removed from the fields growing awned varieties.
Length and
diameter grading of seed rice has been extremely useful in removing the larger
diameter red rice grains from the seed of long-grain varieties. The use of such
graders is important in controlling red rice. In the medium-, and short-grain
varieties, the only means of red rice control is the use of seed and land which
is free of red rice because no method of separation has as yet been devised.
The propagation of seed containing red rice soon results in a wild infestation
of the soil with red rice strains and further complicates the maintenance of
pure seed.
Field inspection
of seed rice fields by the Seed Certifying Agency is carried out 5 to 6 days
before harvest time to establish the varietal purity and identity of seed rice
and to note the degree of infestation with red rice, diseases and pests. Where
required, one additional rogueing may be recommended. Field inspection together
with laboratory analyses of seed samples are used for further seed
certification. In order for the rice to be eligible for certification, the seed
rice has to satisfy specific requirements and standards, which are available
from an official certifying agency. In general, these requirements deal with
application procedures, field and harvest inspections, post-harvest seed
movement, seed processing and sampling. All rice-growing areas use these
standards as the minimum requirements for seed rice.
The Time and Method of Harvesting Seed Rice
The time and
method of harvesting seed rice are both important as they influence seed
quality. The practice of water management in seed crops is equally important.
Drying the fields for harvesting requires the close attention of the grower.
Care should be taken when drying a field that the water recedes gradually,
e.g., at a rate of 1 cm per day. Day-to-day observation has to be carried out
over soil, which is drying in areas where rice seed is not dormant and able to
swell and germinate in the panicle. If this is the case, the depth of water in
the rice paddy should be lowered immediately to a minimum and, in low-lying
areas, withdrawn completely. To be of high quality seed rice must be harvested
at the proper stage of maturity. If the seed crop is cut when immature, field
yields are reduced and the breakage in threshing is excessive because of the
light and chalky kernels. If the seed crop is left in the field until overripe,
the kernels may check.
The difference
in moisture between the inside and the outside of the kernel is said to be the
cause of checking, or shattering of the grain. When too much moisture is
removed due to high temperatures, stresses and strains occur in the kernel
which result in the microcracking of kernels. The checking of rice depends also
on the shape of the grain, the degree of maturity, the variety, and growing
conditions, but the moisture content still remains the decisive factor. Insofar
as the checking of rice is not only the result of the outside (weather)
factors, but also of the mechanical impact it receives during threshing,
cleaning, artificial drying and grading, it is best to employ a method of
harvesting that will result in seed with minimum damage percentage. Two-staged
threshing from the windrow is the preferred method during harvesting seed rice
to reduce mechanical damage. The combine threshes about 80 to 85 percent of the
grain for seed during the first pass. What is left is threshed during the
second round. The USSR RRI tests confirmed by practical observations of growers
have indicated that the least losses occur with double-stage threshing in which
the speed of the thresher cylinder during the first pass (peg-tooth cylinder
550 rpm and raspbar cylinder 750-780 rpm) is slower than during the second pass
(700 and 1,000 rpm, respectively).
Harvesting
should not be started until 90 to 95 percent of the grain in the panicle are
fully mature. This is established by taking an average sample. Seed rice should
be harvested within the shortest time possible and with a minimum interruption
between cutting and threshing. The normal procedure is to cut rice, let it stay
in the windrow for 3 to 5 days to dry, and then thresh it from the windrow. Leaving
the windrows in the field is unadvisable because of adverse weather factors
that may cause the grain to check and lower its quality. Where the two-staged
harvest method is used for different varieties, threshing should by all means
be done with thoroughly cleaned combines. To keep varieties segregated use is
also made of direct combining where the rice plants are not very badly lodged
and the grain yields do not exceed 5 t/ha. In such cases the drying of the
grain can be promoted by applying such chemical desiccants as magnesium sulfate
which has proved useful in seed fields in testes conducted in various rice
areas about the country. Spraying magnesium chlorate at 25 kg/ha hastens the
drying of the grain and straw by 10 to 12 days. This practice prevents lodging,
reduces by 10 to 15 percent the checking of kernels, and permits direct
combining. No grower, however, should use a desiccating material on the
maturing seed crop until he has checked its legal status with reference to
chemical residue tolerances.
Rice Culture
Rice in the
Soviet Union is an artificially irrigated lowland crop seeded directly onto the
check. Nursery transplanting is not practiced.
Modern cultures
of rice in this country rely on the policy of ever increasing rice production
based on the use of engineered rice systems, mechanization, fertilization, and
the latest advances in agricultural sciences and practical rice farming.
Each of the country’s rice
producing areas has incorporated practices of growing and harvesting rice,
which assure high yields (6-7 t/ha) of good-quality paddy rice.
Crop Rotations
In most rice
growing farms crops are rotated because under continuous cropping with rice the
soil becomes depleted in fertility and organic matter. The resulting
deterioration of the physical condition of the rice soil makes cultivation
difficult and the soil becomes infested with weeds and diseases that reduce the
yield and quality of the rice grain.
Proper choice
and establishment of a rotation program is very important for maintaining high
and stable production, controlling weeds and red rice, increasing the
irrigation water and land use efficiency, as well as the use of farming
machinery and labour. Rice rotations help maintain and improve soil tilth and
productivity between rice crops, provide nutrious forage for livestock on the
rice farms and increase the total agricultural output per hectare of riceland.
The preferred system of cropping for any farm depends on the soil type, local
climatic conditions, and economic considerations. In any case, both the
riceland and rice grower should benefit from crops rotated with rice.
Rotational crops are selected so as to help eradicate weeds, reduce populations
of injurious pests, control diseases, and lower production costs.
Although the biology
of rice makes it superior to other crops in that it responds well to repeated
or continuous cropping, rice in this country is rotated with other crops for
the reasons discussed earlier. Rice rotations are also feasible because the
increase in rice yields, despite a smaller proportion of cropland in rice each
year due to rotation, is sufficient to maintain or even increase the total rice
production on rice farms. A high and stable yield of rice under continuous
cropping can be, however, obtained only with heavy application of commercial
fertilizers. The USSR Rice Research Institute has reported that the 27-year
average yield of rice grown in a six-year rotation was by 1.73 t/ha more than
when rice was grown continuously. Rotating rice with other crops is 1.5 times
more economical than maintaining a continuous rice culture. In establishing a
cropping system, a four-year rotation of rice gave 0.45 t/ha, or 10 percent
more rice than the first yield. The yields of rice declined 0.47 t/ha within
the same period under continuous cropping. In rotation experiments in the USSR
Far East, the yield of rice in a seven-year rotation system was found to be 1.5
times that of rice under continuous cropping. Similar results were reported
from the Uzbek SSR Rice Research Institute.
Continuous
planting of lands to rice leads to heavy infestation of riceland with the
rice-culture related weeds, to the detriment of the soil's physical condition
and depletion of its fertility.
The beneficial
effect of crop rotation on the rice yields can be attributed to many factors.
First, rotations enrich the plow-line soil layer in organic matter and
eliminate aquatic and other injurious weeds. Rotations facilitate oxidation of
the chemically reduced nutrients, improve porosity, reduce the bulk mass by
improving soil texture (less amount of particles smaller than 0.25 mm). They
are also helpful in controlling insects and diseases and providing better
opportunities for surface levelling through timely operations. On commercial
rice farms, rotations ensure comparatively high and stable grain yields.
Rice rotations
in this country were first used in the old Kuban delta land, which were
formerly overgrown with boggy-reed vegetation. An 8000 ha area had been
developed for rice and six-and seven-year rotation systems were tried on its
low-productive, overmoist and partly salinized soils. In the years 1971-75,
average yields on the rice farms
Table 1. Rotation vs Continuous
Cropping (the Kuban area)
Rice rotations
have come into use also in the new Kuban delta ricelands to benefit the rice
growers with 5.5 t/ha and more rice, which is 1-1.5 tons more than the average
yields on the neighbouring farms where rotations are not yet customary.
Cropping systems
or rotations have been used by many rice farms in other rice producing areas of
the Soviet Union just to demonstrate that crop rotation is essential to ensure
rice yields of about 6.0 t/ha, or even more.
Cropped Land Structure
Under a rotation
program it is sought to use a maximum of cropland in rice following crops that
are proven the best predecessors, or forecrops. Such crops for rice are those
that improve soil productivity and help the rice grower obtain good returns
from a hectare of cropland. For this purpose, the irrigated ricelands should for
the greater part of the year be preferably used for raising high-yielding
crops. Since livestock has been extensively developed in most rice-growing
areas, such crops are grown basically for feed purposes. In this way, crop
rotations are a useful tool in matching up the cultivation of rice and
livestock raising.
Usually the rice
systems are designed and engineered for a particular rotation pattern. The
choice for a cropping pattern is therefore very important, the determining
factors being agricultural specialization, soil type, water and drainage
conditions in the locality, and the agronomic function of the rotation system.
The idea of crop rotation implies that crops be periodically changed, e.g.,
flooded rice is followed by a dryland crop. Such alternation of crops is
mutually beneficial because it helps eliminate the deteriorative effect on the
rice soil of extensive floods by allowing the soil to dry out when it is in a
dryland crop. The cropping systems should be selected so that the proportion
and the order of crops in the cropland are easily adaptable to different
economic situations without readjusting the irrigation facility layout.
Research and farming have proved that long-time rotations, such as the seven-,
eight-, and nine-year rotational programs, are most suitable in this respect.
Of the numerous
long-time cropping systems, the eight-year rotation with perennial grasses and
seeded or cultivated fallows is preferred as the most flexible one. Under such
a cropping pattern, 62.5 percent of the land is used for rice, this proportion
being easily increased to 75 percent when necessary. The rice soil benefits
from this system in receiving a double amount of organic matter, first from
turning under the perennial grasses, then from the annuals. In addition, the
eight-year rotation system provides better opportunities for the basic
land-forming and levelling operations in each field check. In most rice
producing areas, this cropping pattern has been the basis for design and
construction of new riceland developments. Also, other scientifically-grounded
cropping systems involving rice for various periods have been in use on rice
farms of other locations in the Kuban delta lands.
The Krasnodar Territory
Many rice farms
use the eight-year rotation with the following orders and frequency of crops:
first two years, perennial grasses (alfalfa, clover); third to fifth year,
rice; sixth year, seeded fallow, followed by two annual crops of rice (with
62.5 percent of the land being used for rice; 25 percent, for perennial
grasses; and 12.5 percent, for cultivated fallows, under the system). About
one-fourth of the cropland in the Kuban delta is in a seven-year rotation:
first and second year. Perennial grasses (alfalfa, clover); third to fifth
year, rice; sixth year, cultivated fallow, and seventh year, rice; or first
year, cultivated fallow; second and third year, rice; fourth year, other grain
crops overseeded with perennials; fifth year, grasses, and sixth and seventh
year rice (with 57.1 per cent of land in rice, under the system). Where the
long-time rotation is impracticable, but the agronomic practices are advanced,
and labour and power resources are plentiful, the rice growers choose to use
short-term cropping systems, such as the three year rotation: first year,
seeded fallow and second and third year, rice (with 66.7 percent of the
cropland in rice); and four-year rotation: first year, cultivated fallows and
three years in rice, i.e. three-fourth of the time the land being used for
rice, under the system.
The Don Piver and Cis-Caspian Lowland
Depending on
local conditions and economic considerations, rice growers here may choose
between six-, seven-, and eight-year rotation systems.
In a six-year
rotation, the frequency of crops is: first and second year, perennial grasses;
third and fourth year, rice; fifth year, seeded fallow (spring grain crops) and
sixth year, rice (with 50 percent of land in rice, 33.4 percent in perennial
grasses, and 16.6 percent in seeded fallows). Also, row-crops and pulses may be
fallow-grown in some localities.
The seven-year
cropping systems recommended for these areas are similar to those used by the
rice-growing farms in the Northern Caucasus. The fallow-grown crops may vary
with the locality from winter wheat, pulses or spring barley in eight-year
rotations (with 62.5 percent of land in rice) to vegetable crops, in seven-year
rotations.
The USSR Far East
In the Monsoon
climate of the Far East the cropping patterns vary. The eight-year rotation may
have a different order of crops depending on the depth of snow pack in the
winter. Thus, in localities where snow cover is permanent, an eight-year
rotation may be: first to third year, rice; half of the fourth year, green
manure crop, the other half — maintenance of the irrigation facilities; fifth
and sixth year, rice; seventh year, barley or oats over-cropped with clover;
eighth year, clover (with 62.5 percent of the land in rice). Where snow is
marginal, the order and frequency of crops is: first to third year, rice; half
of the fourth year, green manure crops, the other half — maintenance of the
irrigation facilities; fifth and sixth year, rice; seventh year, cultivated
fallow; and eighth year, forage crop, the percentage of land in rice being the
same. In other localities, recommendations are for a seven-year rotation as
follows: first year, grain crop; second year, feed crop; third and fourth year,
rice; fifth year, green manure crop; sixth and seventh year, rice (with 57
percent of cropland being used for rice). A six-year rotation allows for one
year in grain crop, two years in rice, one year in soybeans for green manure
and two years in rice (with 66.7 percent of land in rice). The practice for
newly developed ricelands has been a four year rotation consisting of three
years in rice followed by half a year of green manure crops and the other half
used for maintenance of the irrigation facilities (with 75 percent of land in
rice, under the cropping system).
The Ukraine, Uzbekistan, and Southern Kazakhstan
With allowance
for the local traditions and climate, the cropping patterns are essentially the
same but may vary in length from four to nine years, also in the order of crops
and in the proportion of land in rice, which may range from 43 to 66.7 percent.
Whatever the order and frequency of crops in rotations, rice growers have to
follow the general tendency of crops in rotations, rice growers have to follow
the general tendency of allotting a maximum and economically feasible
proportion of the land to rice as a staple culture, and grow catch-crops on it
in between rice croppings.
Intensified Cropping Systems
Because of the
high cost of land development for rice, one way to ensure good returns from a
hectare of irrigated land is by putting the riceland to intensive agricultural
use. Considering the limited geography of rice in this country, another way is
to extend the acreage for rice in a rotation in addition to increasing the
yield of rice through improved agronomy and superior varieties. Research on
rotating rice with other crops has proved it possible to repeat rice cropping
(up to four years) in the same field. Obtaining high and stable yields under
such a system of cropping requires periodic incorporation into the soil of
organic matter, optimum applications of fertilizer, good water management,
sufficient treatment of the field with herbicides, and adequate agronomic
practices. Rotational experiments conducted by the USSR Rice Research Institute
indicate that the yield and gross output of rice can be increased through using
rotations, making better use of perennial grasses, increasing to more than
three years the length of repeated cropping of rice after perennial grasses,
and through growing catch-crops between rice croppings.
The eight-year
rotation system developed by the researchers for the Kuban delta ricelands can
be considered as intensified rotation with 75 percent of land in rice. The
coefficient of land use under this system increases from 1.25 to 1.75 due to
growing catch-crops and better use of perennial grasses.
Time 2. Rotation of Rice with and
without Catch-Crops
Forecrops
The growth of
agricultural plants and cultural methods used for soil cultivation, and
particularly application of water and fertilizers, cause various changes in the
physical, chemical and biological properties of the soil. This in turn affects
the growth and development of crops that are grown on the same field the
following years by increasing or decreasing their yield. The knowledge of how
the individual species or groups of plants may influence the crop grown in
alternate years is very important for appraising these plants or species as the
forecrops, for setting the proper order and frequency of crops in a
rotation.
It has been
proved by many tests and practical rice farming that perennial legumes,
fallow-grown annual legumes and green manure crops, leguminous-gramineous
mixtures and cruciferous plants, and catch-crops grown for seed and green
manure are best for growing in rotations ahead of rice.
For other
rice-growing areas, the crops preceeding rice in rotations are essentialy the
same. In addition, sweet or sour clover, crimson clover mixed with berseem or
Egyptian clover are sown in Kazakhstan, Uzbekistan and Turkmenia. The Sudan
grass and spring wheat are grown in fallow fields and as catch crops in the
Ukraine and Kazakhstan; while corn (maize), sorghum, joughara mixed with mung
beans, sweet clover and vetch-oats mixtures are sown in Kazakhstan, Uzbekistan
and Tajikistan.
The rice soil
benefits much from alfalfa and clover if grown for two years. The grasses
improve the physical condition of the soil, increase the content of organic
matter and soil productivity. Perennials facilitate the conversion of almost
insoluble phosphorus compounds into readily soluble ones whose quantities tend
to increase with the age of grasses. With a two-year old grass cover, the soil
has a maximum of available phosphates. In rice rotations the total yield of
alfalfa hay (four cuts) may reach 8-10 t/ha with the cost of one feed unit much
lower than that of annual legumes. High yield of alfalfa in rice rotations is
due to good agronomic practices including check-flood irrigation or sprinkling
and fertilizer applications.
The beneficial
effect of alfalfa on the rice soils is higher when the two-year old grass is
left over winter to be turned under the following spring after the first cut of
hay. In this case, it gives additional 25-30 t/ha of green matter (5 tons on
dry matter basis) before the field is sown to rice. The method of turning under
alfalfa in spring has become customary with the rice farms in the Kuban rice
areas ensuring stable yield of good-quality hay in addition to 5 t/ha of early
of mid-season rice each year, and increasing the organic matter in the soil in
the form of roots and other plants debris. The higher the yield of perennial
grasses grown ahead of rice in rotation, the higher their beneficial effect on
the rice soils and, consequently, on rice yield. Grasses, therefore must be
given the best agronomic care including seasonal irrigation and fertilizer
treatments combined with soil slitting to produce highest yields of hay already
in the first year.
Modern agronomic
practices and adequate timing of optimum nitrogen and phosphorus fertilizer
applications make it possible to maintain and sometimes increase the yield of
rice grown three years continuously after grasses.
The yields of
rice in an eight-year rotation depending on the forecrop were as follows (the
data of the USSR RRI).
Practical rice
growing in the Kuban ricelands showed that alfalfa grown for two years ahead of
rice and plowed under in the spring before seeding rice gives assured 5.0-5.5
t/ha of rice grain, and with fertilizers, up to 6.0-7.0 t/ha. Similar yields of
rice in grassland broken at fall are attainable only with the application of
90-100 kg/ha of nitrogen fertilizers and phosphates (P2O5).
Fallowing
The chief aim of
fallowing fields is controlling weeds; check land leveling, and reshaping and
maintaining water structures. But because the land development for rice is
costly, it is unwise to allow the land to lie idle, and hence pure fallowing is
not encouraged. The fallow fields are therefore seeded or cultivated which
permits the chief aim of fallowing to be achieved plus the fallow-grown crops
additionally gathered.
Seeded or cultivated fallows are
fields used for growing various agricultural crops which when ripe leave fields
free from plants soon after harvest for the land-levelling operation. such
crops in the Northern Caucasus are winter wheat mixed with winter peas or vetch
grown for hay or green chop, spring vetch mixed with oats, winter and spring
peas mixed with oats or barley, and winter barley. The fallow-grown crop in the
Lower Volga rice farms is mostly winter rye mixed with vetch for green chop. In
the Far-East ricelands such crop is soybeans.
The use of
mineral fertilizers for the fallow-grown winter crops is mandatory in all the
rice producing areas. The rates vary with the area and soil productivity. The
soils in the Kuban delta lands require 120 kg N in addition to 90 kg P2O5 per
hectare applied as basal fertilizer during the fall plowing for grains in
pulses. Nitrogen applications are split into 90 kg/ha at seedbed preparation
and 30 kg/ha as an early dressing.
For early spring
crops, such as barley, wheat, peas and oats mixed with vetch and peas, the
fertilizers are applied at seedbed preparation, or at harrowing.
The yields of vetch and oat
mixtures sown in fallows for hay are about 5 t/ha; winter wheat and peas
produce by early spring 3 to 4 t/ha and winter peas sown in autumn produce up
to 3 t/ha of nutritious green matter.
All these crops
are however susceptible to excess moisture. Crop failures may result from too
much water held in checks after heavy rainfall and cloudburst unless adequate
drainage is provided.
The choice and
composition of fallow-grown crops relies on the economic considerations,
availability of seeds, and the possibility for annual land-levelling in the
checks, which is a key operation for obtaining high rice yields the following
season. In selecting and allotting lands to the accompanying crops of rice
rotation and fallows, the physical condition of the flooded soils is
particularly important. Alfalfa, barley, corn and peas do not grow well where
drainage is poor and the water table high. Their yields are low from excess
water and poor thin stands. Adequate drainage is therefore the only remedy from
water logging and inundation of rice fields and the adjacent areas, which are
in dryland crops. Of the crops, which can tolerate high ground waters, crimson
clover, berseem (Egyptian clover), and mung beans are the most tolerant.
Benefits to the
staple rice culture from cultivated fallows in the rotation are high only with
good weed control, proper grading and levelling of land, and increased organic
matter in the soil due to fallow-grown annual legumes and grasses. The
intensive use of land through seeded fallows makes possible double cropping of
riceland so that two crops are harvested the same year, provided all operations
are expertly timed.
Catch-crops
Double cropping
implies growing catch-crops for use either at fall or early next spring as feed
or green manure the same year after the main fallow-grown crop is harvested,
field levelled and given the semi-fallow tillage. Growing catch-crops is also
important for improving soil productivity and rice yield. The name catch-crop
applies to crops grown the same year following the staple crop and intended for
feed or green manure. They are also known as stubble crops. The term is also applicable
to crops sown in the spring into the cover crops to keep growing still for some
time after the cover crop is harvested. Such crops are also called the
companion or nurse crops; the name applies to crops sown in summer or in the
fall following the staple crop and harvested for feed purpose the following
spring before a main crop is sown, and known as the wintering crop; and also to
crops sown on fields free from the previous crop harvested early in season for
green chop, sillage or hay, and sometimes called the postharvest crops which
elsewhere can be grown as the main crop.
The agricultural
plants selected to be grown as catch crops should be high yielding and early
maturing recommended for this or that area, and well adapted to heavy and
periodically flooded soils. Among such crops are pulses (winter and spring
vetch and peavine), winter rye, winter wheat, barley, oats, spring rapeseed,
all sown in pure or mixed stands.
In the Northern
Caucasus and the Lower Volga rice areas the fallow-grown catch crops are sown
in the summer or fall and thus are called summer crops. The same crops to be
grown in rice fields are sown as winter crops. In the rice producing areas of
the USSR Far East the catch crop is soybeans (when grown in fallows it is for
green manure, although soybeans can be grown for grain).
Winter rye is
good as a catch crop. Some of its winter varieties are winter-hardy and shoot
out well early in the spring at low temperatures (close to zero), producing
fairly good yield of nutritious green matter, so valuable early in the spring
for its vitamins.
In many
rice-growing areas of this country and particularly in the Cis-CaspianNI200 Lowland,
rotational crops are grown in saline soils. In such cases, adequate drainage
and importation are necessary to avoid water logging, inundation and
salinization of the land in accompanying crops and grasses that are adjacent to
rice fields on the one hand, and make the best use of the rotation, on the
other. Of the accompanying crops, peas, oats and corn are less tolerant to
salts than are rye, wheat, sorghum, and particularly alfalfa. Gourds and melons
tolerate better high concentrations of salts. Soils moisture content is an
important regulator of the degree of salt tolerance of the rotational crops. The
higher the moisture content, the more tolerant the plants to salinity during
their early development.
To provide high
and stable yield, each rotational crop in a rice cropping system should be
grown under optimum agronomic conditions. It has been established that the rice
yield to a large extent depends on the productivity of the preceeding crops.
Thus, yield or rice following one-year alfalfa, depending on its crop of hay,
was as follows:
Good timing of
catch crops is also important in a rice rotation. It is advisable that in the
rice fields, which are planned the following season for catch-crops, rices are
early-maturing and sown in the current year as early as possible. In that way
the crop of rice is ready to harvest much early giving the grower time enough
to prepare the land for catch crops of the following year.
Land Preparation
Tilling soil for
rice is not much the same as tilling for other cereals and dryland crops. Its
principal aim in rice production is to obtain high yields of rice through
improving the rice soil and taking advantage of its potential productivity.
While the
dryland crops require soil nutrients in the oxidized form, the rice plant
benefits more when the nutrients are chemically reduced or deoxidized. The
dryland crops require that the capillary-noncapillary porosity ratio
(determined by the water-stable soil structure and soil moisture brought to
capillary capacity) be optimum, while this soil parameter for rice is
practically for rice is practically unimportant.
Nutrition of the
rice plant is in large measure assured by inundation during part of all of the
growing period. Flooding is very much essential for optimum grain yields that
are why the ideal soil types for rice production are those that conserve water.
Most rice soils, often referred to as heavy soils because of their high clay
and silt content, present special soil management problem that are overcome
through soil cultivation practices intended also to help make the best use of
the natural soil potential. These measures include tillage and seedbed
preparation, maintenance of organic matter and soil texture, drainage for
successful mechanized rice operations, cultivation of other crops in rotation
with rice, fertilizer application, use of green manures, and weed control.
Soil tillage
practices vary from place to place depending on soil type, climatic conditions,
crop that preceedes rice in rotation, physical condition of the soil, character
and degree of field infestation, herbicides used and other factors. Tillage in
rice production pursues many purposes, which are generally aimed at:
(1) Forming a sufficiently deep and
biologically active plowline layer by working the field several times over with
various types of plow;
(2) Creating conditions in the plow-line
that help immobilize soil nutrients, i.e. regulate oxidation and reduction
through loosening, drying and aerating of soil;
(3) Wetting the rice fields that are to be
sown at early dates and to a greater depth so as to establish the moisture
content sufficient to bring about emergence of rice seedlings without
additional flush-irrigation;
(4)
